(452b) Understanding the Mechanism of Simultaneously Oleophobic/Hydrophilic Surface: When a Nanometer-Thick Polymer Coating Meets An Attractive Solid Substrate

Understanding the mechanism of simultaneously oleophobic/hydrophilic surface: When a nanometer-thick polymer coating meets an attractive solid substrate

Lei Li¹, Yongjin Wang¹ and Jianing Sun²

1. Department of Chemical and Petroleum Engineering, University of Pittsburgh, 3700 O’Hara Street, Pittsburgh, PA 15261

2. J.A. Woollam Co., Inc., 645 M Street, Suite 102, Lincoln, NE 68508

Simultaneously hydrophilic/oleophobic surfaces are highly desirable in many important applications.However, such wetting behavior are not expected for traditional surfaces since previous experimental results showed that, in general, a solid surface is more wettable to oil than to water. Interestingly, a couple of recent studies showed that it is possible to achieve simultaneous “oleophobicity/hydrophilicity” by applying some nanometer-thick polymer on some solid substrate. The underlying mechanism, which is critical to fabricating “oleophobic/hydrophilic” surfaces, remains unclear to date. In the current presentation, we report our recent experimental work in uncovering the mechanism of “hydrophilic/oleophobic” surfaces. First, another “hydrophilic/oleophobic” surface, which is fabricated by dip-coating a nanometer-thick perfluoropolyether on a silicon wafer and shows higher water contact angle (WCA) than hexadecane contact angle (HCA), has been identified. Second, it was found that the HCA on this surface changes significantly with time, which indicates that the observed “oleophobicity/hydrophilicity” is kinetic in nature. Third, the effect of the chemicals structure of both polymer and substrate on the peculiar wetting behavior has been studied. The experimental results suggest that the interaction between the nanometer-thick polymer and the substrate is the key to the appropriate size of the intermolecular “hole” in the polymer layer, which in turn determines the kinetics of the penetration of water and hexadecane through the polymer nanofilm. Only when the “hole” size is appropriate so that the penetration rate of hexadecane is much slower than that of water, the peculiar “hydrophilic/oleophobic’ wetting performance is observed. On the basis of these findings, the design criteria to fabricating hydrophilic/oleophobic surfaces have also been proposed.